This invention relates to a motor vehicle electrical system, and more particularly to a series-pass protection circuit that protects an electrical load from over-voltage damage.
It is well known that motor vehicle electrical systems are subject to over-voltage under a number of different conditions. For example, jump-starting the engine with an excessive supply voltage subjects the electrical loads to the excessive voltage as well. Also, the ignition voltage is subject to transient surges during so-called load dump events when the vehicle storage battery is disconnected from an engine-driven alternator. In such case, the alternator output voltage can rise well above the nominal charging voltage before the voltage regulator can remove the alternator field winding excitation. For this reason, vehicle electrical systems are typically equipped with shunt suppression devices (Zener diodes or MOVs, for example) that clamp the load voltage to a predetermined value. However, in some cases, the transient over-voltage energy is too high to clamp with shunt devices; in these cases, a series-pass suppression device (such as a linear transistor) can be used. However, this approach is ordinarily ruled out in applications requiring both high operating current capability with low series impedance, and low quiescent keep alive current. These requirements occur, for example, in a heavy-duty electrical system, where the storage battery supplies keep-alive (quiescent) current to the engine control module when the ignition key is off, and the engine-driven alternator supplies high load current during engine operation. Accordingly, what is needed is a series-pass over-voltage protection circuit that has both high operating current capability with low series impedance and low quiescent current draw.
The present invention is directed to an improved series-pass over-voltage protection circuit for a motor vehicle electrical system, including first and second parallel connected power paths that respectively provide low current capability with low quiescent current drain, and high current capability with low on-resistance. The first power path, embodied by a P-channel MOSFET, supplies pass-through battery current to quiescent loads such as an engine control module during periods of vehicle inactivity. The second power path, embodied by at least one N-channel MOSFET, is activated by a gate voltage power supply during periods of vehicle activity. The second power path is configured to limit the circuit output voltage to a higher value than the first power path, so that the over-voltage protection during periods of vehicle activity is accomplished by the second power path.